[Tile-serving] [osm2pgsql-dev/osm2pgsql] Add grouped-linemerge generalization (PR #2482)

[leijurv]

@leijurv commented on this pull request.



> +    CASE WHEN ST_X(ST_StartPoint(l."{geom_column}")) = n.x
+          AND ST_Y(ST_StartPoint(l."{geom_column}")) = n.y
+         THEN ST_X(ST_EndPoint(l."{geom_column}"))
+         ELSE ST_X(ST_StartPoint(l."{geom_column}")) END,
+    CASE WHEN ST_X(ST_StartPoint(l."{geom_column}")) = n.x
+          AND ST_Y(ST_StartPoint(l."{geom_column}")) = n.y
+         THEN ST_Y(ST_EndPoint(l."{geom_column}"))
+         ELSE ST_Y(ST_StartPoint(l."{geom_column}")) END
+    FROM nodes n
+    JOIN {src} l
+      ON {group_join}
+     AND ( (ST_X(ST_StartPoint(l."{geom_column}")) = n.x
+        AND ST_Y(ST_StartPoint(l."{geom_column}")) = n.y)
+       OR (ST_X(ST_EndPoint(l."{geom_column}")) = n.x
+        AND ST_Y(ST_EndPoint(l."{geom_column}")) = n.y) )
+     AND {where}

I think this is true already, I've looked at it in `EXPLAIN ANALYZE` and `WorkTable Scan on nodes n (rows=43 loops=319)` shows each iteration joins only ~43 frontier rows, not the 13613 accumulated. If it were re-joining the whole set, that scan would show thousands of rows. It works because I am doing `WITH RECURSIVE ... UNION` rather than `WITH RECURSIVE ... UNION ALL`.

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